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& Planetary Science 45, Nr 10–11, 1657–1667 (2010) doi: 10.1111/j.1945-5100.2010.01107.x

Almahata Sitta—Fragment MS-CH: Characterization of a new type

Marian HORSTMANN1*, Addi BISCHOFF1, Andreas PACK2, and Matthias LAUBENSTEIN3

1Institut fu¨ r Planetologie, Wilhelm-Klemm-Str. 10, 48149 Mu¨ nster, 2Geowissenschaftliches Zentrum, Universita¨ tGo¨ ttingen, Goldschmidtstrasse 1, 37077 Go¨ ttingen, Germany 3Laboratori Nazionali del Gran Sasso, I.N.F.N., S.S.17 ⁄ bis, km 18+910, I-67010 Assergi (AQ), Italy *Corresponding author. E-mail: [email protected] (Received 19 February 2010; revision accepted 07 August 2010)

Abstract–Among the several hundred, mostly small fragments, recovered within the Almahata Sitta strewn field, one fragment (MS-CH), weighing 5.68 g, was detected that represents a new type of chondritic meteorite. The detection of short-lived cosmogenic radionuclides clearly indicates that this chondrite fragment results from a fresh consistent with the Almahata Sitta event in October 2008. The fundamental mineralogical characteristics of the Almahata Sitta fragment MS-CH can be summarized as follows: (1) the almost equilibrated has high Fa contents of about 36 mole%. The fragment is of petrologic type 3.8 ± 0.1; (2) the metal abundance of the rock is on the order of 2.5 vol%; (3) the mean size has been determined to be roughly 450 lm; (4) point-counting and imaging indicate that the matrix abundance is approximately 45 vol%; (5) Cr-spinels have much lower TiO2 concentrations than typical spinels within R ; (6) calcium-aluminum-rich inclusions are spinel-rich and severely altered having abundant Na- and ⁄ or Cl-rich alteration products. Spinel also contains significant concentrations of Fe and Zn; (7) magnetites and platinum-group element-rich phases (sulfides, tellurides, and arsenides) characteristic of both R and CK chondrites were not found in fragment MS-CH; and (8) the mean oxygen isotope composition of three small fragments of Almahata Sitta MS-CH is d17O = +4.35&, d18O = +4.94&, and D17O = +1.76&. The oxygen isotopes relate MS-CH to R chondrites. No established chondrite group having all these characteristics exists.

INTRODUCTION on these findings, Bischoff et al. (2010a, 2010b) suggested that Almahata Sitta is not only a ureilitic On October 7, 2008, the 2008 TC3 was meteorite but also a spectacular containing detected in space about 19 h before it impacted Earth in clasts of chondritic and achondritic lithologies. the Nubian Desert of northern . This was the first Among the small meteorite fragments recovered time ever that an asteroid was observed and studied within the Almahata Sitta strewn field, a fragment (MS- before its fall on Earth. Several hundreds, mostly small, CH) with a mass of 5.68 g was detected representing a meteorite fragments were recovered and were new type of chondritic meteorite. In this study, we have subsequently named as the Almahata Sitta meteorite. characterized the mineralogical properties of this unique Almahata Sitta has been classified as an anomalous chondrite type and also present the oxygen isotopic polymict (Jenniskens et al. 2009) and described composition and data on short-lived radionuclides mineralogically by Zolensky et al. (2009) and Herrin analyzed within the rock. et al. (2009). In a detailed study, 40 small pieces from different SAMPLES AND ANALYTICAL TECHNIQUES quite fresh fragments collected within the Almahata Sitta strewn field were investigated and about 20 Almahata Sitta fragment MS-CH is a relatively different lithologies were found (Bischoff et al. 2010a, fresh, complete meteorite fragment of 5.68 g found in 2010b; Horstmann and Bischoff 2010a, 2010b). Based the Almahata Sitta strewn field in July 2009. A small

1657 The , 2010. 1658 M. Horstmann et al. slice was selected for the preparation of three tiny thin Germanium (HPGe) detectors, in ultra low background sections (PL09301, PL09302, and PL09303) and a 5.1 g configuration (25 cm of lead and an inner liner of 5 cm sample was used to measure the short-lived cosmogenic copper, Rn suppression system, inside an underground radionuclides at the Laboratori Nazionali del Gran laboratory with 1400 m rock overburden). The counting Sasso (Italy). Small grains from three different areas of efficiency was calculated with a thoroughly tested Monte the fragment (with a mass collectively <5 mg) were Carlo code. The 5.1 g sample was measured during the prepared for oxygen isotope measurements at time period from October 13 to 18, 2009. Universita¨ tGo¨ ttingen (Germany). The mineralogy and texture of the fragments were RESULTS studied by light and electron optical microscopy. A JEOL 6610-LV scanning electron microscope (SEM) Hand Specimen with an attached Oxford Instruments EDS system was used to resolve the fine-grained textures and to analyze The original individual specimen had a weight of the mineral constituents. As analytical standards we 5.68 g and was covered by a black fusion crust. The used olivine (Mg, Fe, and Si), jadeite (Na), plagioclase black fusion crust is still visible in the remaining main (Al), sanidine (K), diopside (Ca), rutile (Ti), chromium mass (Fig. 1) after some parts of the fragment have oxide (Cr), rhodonite (Mn), (Fe, Ni, and S), been broken off or cut. The cut surface is grayish and Co-metal (Co). having abundant light rounded inclusions (mainly Most quantitative mineral analyses were obtained and chondrule fragments) embedded within using a JEOL JXA 8900 electron microprobe operated at abundant matrix. Metals at the surface have already 15 keV and a probe current of 15 nA. Natural and obtained a very slight brownish staining due to synthetic standards of well-known compositions were oxidation reactions with terrestrial humidity between used as standards for wavelength dispersive spectrometry. the fall and recovery of the rock fragment. These are jadeite (Na), sanidine (K), diopside (Ca), disthen (Al), fayalite (Fe), chromium oxide (Cr), Mineralogy of the Unique Chondrite Fragment MS-CH pentlandite (Ni, S), hypersthene (Si), San Carlos olivine (Mg), rhodonite (Mn), rutile (Ti), Co-metal (Co), Texture chalcopyrite (Cu), tugtupite (Cl), and willemite (Zn). The In thin section, MS-CH shows a well-preserved matrix corrections were made according to the Uq(z) chondritic texture (Fig. 2a). Obviously, the sample has procedure of Armstrong (1991). a relatively high abundance of matrix. Based on The oxygen isotope composition of three point-counting on photomicrographs, a matrix fragments (MS-CH-1: 0.88 mg; MS-CH-2: 2.139 mg; abundance of approximately 45 vol% has been MS-CH-3: 0.754 mg) of Almahata Sitta MS-CH was determined, which is in the range of CO, CV, CK, and analyzed by infrared laser fluorination (Sharp 1990) in R chondrites (e.g., Grossman et al. 1988 and references conjunction with isotope ratio monitoring mass therein; Kallemeyn et al. 1991; Rubin and Kallemeyn spectrometry. Samples were fluorinated in an 1993; Bischoff 2000) and significantly higher than within atmosphere of approximately 20 mbar with purified F2 ordinary chondrites (e.g., Grossman et al. 1988 and gas. Excess F2 was reacted to Cl2 in a heated NaCl references therein). As coarse-grained constituents, trap (NaCl + 0.5 F2 fi NaF + 0.5 Cl2) and Cl2 was fragment MS-CH contains abundant chondrules and cryogenetically separated from sample O2. Sample O2 chondrule fragments, as well as metals, sulfides, and was collected on a molecular sieve trap from where it some calcium-aluminum-rich inclusions (CAIs), which was transported by a He carrier gas flow via a second will be described below. No indication was found that molecular sieve and a 5A gas chromatography column the minerals in the clast have suffered reduction by into the source of a ThermoElectron MAT253 triple reaction with the ureilite host. collector gas mass spectrometer. Accuracy and precision were usually at ±0.2& for d18Oand Chondrules ±0.06& for D17O. Terrestrial rocks and minerals Most chondrules or chondrule fragments are (290 analyses) were used for the definition of the porphyritic, but all typical types of chondrules occur terrestrial fractionation line with a slope of 0.5250 ± (Fig. 2). Barred-olivine chondrules appear to be more 0.0007 (1r). abundant than within ordinary chondrites (Fig. 2). A The short-lived cosmogenic radioisotopes were mean chondrule size of approximately 450 lm has been measured at the Laboratori Nazionali del Gran Sasso determined based on the analyses of the apparent (LNGS) (Italy) by means of gamma ray spectroscopy. diameter of 44 chondrules. The mean chondrule size is The measurements were performed using high-purity similar to that in R chondrites. Unique chondritic fragment Almahata Sitta MS-CH 1659

Fig. 1. Photomicrograph of the Almahata Sitta stone investigated in this study. The black fusion crust is still visible after some parts of the fragment have been broken off or cut. The grayish cut surface contains light chondrules and chondrule fragments embedded within abundant matrix.

Main Minerals Olivine within chondrules and matrix is mainly Fa35–37. Only a very limited number of were encountered having lower Fa contents. These zoned olivines are large and the olivine with the highest, so far analyzed Mg concentration has a Fa content of 19 mole%. Random analyses of olivines revealed a mean olivine composition of 35.0 ± 3.6 mole% Fa. Thus, the olivine compositions indicate that the rock is almost equilibrated. The olivines do not contain NiO as found in R and CK chondrites (Table 1). On the other hand, the low-Ca pyroxenes are variable in composition (Fs3–26). A small, weak peak in the Fs distribution can be found at about 24–26.5 mole% Fs, but low-Ca pyroxenes with Fs contents as low as 3 mole% exist (Fig. 3). The Cr-spinels have much lower TiO2 concentrations than those within R chondrites. Representative analyses of the main silicates and oxides are given in Table 1. The rock has significant abundances of metals and troilites of about 2.5 and 5 vol%, respectively, as obtained by SEM surface-imaging using the analySIS software (Olympus Soft Imaging Solutions). Sulfides and metals as large as 460 · 360 and 280 · 160 lm, respectively, occur within the studied thin sections. Figure 4d shows an area within MS-CH having a relatively high abundance of metals and sulfides. The Fe,Ni-metal is mainly Ni-rich (Ni: 40 wt%; Co: 2 wt%; taenites). The less abundant has on the order of 7 wt% Ni and 1.5 wt% Co. Fig. 2. Photomicrographs in transmitted light (crossed polars). Representative compositions of metals and sulfides are a) Overview of the Almahata Sitta fragment MS-CH showing given in Table 2. Magnetites and platinum-group element the chondritic texture and the abundance of matrix. b) Typical (PGE)-rich phases (sulfides, tellurides, and arsenides) porphyritic olivine–pyroxene chondrule. c) Fine-grained pyroxene-rich chondrule. d) Barred-olivine chondrule characteristic of R and CK chondrites were not found in fragment. fragment MS-CH. 1660 M. Horstmann et al.

Table 1. Microprobe analyses (except as indicated) of the main phases as well as of chromite and ilmenite. Olivine Pyroxene Plagioclase Chromite Ilmenite Mean (n = 34) Min.* Mean (24) Min. Max. Mean (5) Min. Max.

SiO2 36.5 39.2 55.9 58.1 53.9 66.9 67.4 65.8 0.17 <0.02 Al2O3 <0.01 n.d. 0.38 0.21 0.55 19.6 19.0 19.6 5.7 n.d. Cr2O3 0.05 <0.01 0.43 0.39 0.44 <0.01 n.d. n.d. 56.5 0.09 TiO2 <0.02 <0.04 0.03 n.d. 0.07 <0.02 n.d. n.d. 1.35 54.8 FeO 31.3 18.2 9.4 3.2 17.1 0.89 0.93 1.20 31.5 43.6 NiO <0.02 <0.10 <0.01 <0.01 n.d. <0.01 n.d. 0.04 n.d. 0.04 ZnO <0.03 n.a. 0.03 <0.01 n.d. <0.02 n.d. 0.06 0.82 <0.05 MgO 32.8 42.0 32.3 37.5 26.1 0.26 0.56 0.47 1.84 2.73 MnO 0.40 0.55 0.30 0.17 0.24 <0.01 <0.01 <0.02 0.50 0.70 CaO 0.05 n.d. 0.50 0.21 0.84 1.27 0.96 1.50 0.11 n.d. K2O <0.01 <0.01 <0.01 n.d. n.d. 0.60 0.68 0.55 n.d. n.d. Na2O <0.01 <0.04 0.04 n.d. n.d. 10.4 10.5 10.3 0.11 n.d. Total 101.10 99.95 99.31 99.78 99.24 99.92 100.03 99.52 98.60 101.96 Fa ⁄ Fs ⁄ An 35.0 19.6 14.4 4.57 26.5 6.1 4.7 7.2 Note: All data in wt%. n.d. = not detected; n.a. = not analyzed; n = number of analyses. *EDS analysis normalized to 100%.

Shock Effects Almahata Sitta fragment MS-CH has been affected by shock metamorphism (Sto¨ ffler et al. 1988, 1991; Bischoff and Sto¨ ffler 1992). The olivine shows undulatory extinction and irregular fractures. In about 10–20% of the olivine grains, planar fractures are visible indicating an even higher degree of shock metamorphism. Using the shock classification scheme for ordinary chondrites by Sto¨ ffler et al. (1991), fragment MS-CH is very weakly shocked (S2).

Calcium-Aluminum-Rich Inclusions Several CAIs were found having abundant spinel (Figs. 4a and 4b). Representative mineral analyses of the constituents are given in Table 3. CAI CH-1 is a concentric spinel-rich inclusion with a spinel-rich core surrounded by a porous nepheline-bearing layer (Fig. 4a; Table 3). The inclusion is almost completely rimmed by Ca-pyroxene and in some areas by an additional rim of olivine (Fa36). The spinels have high Fig. 3. Plot of the Fs content of low-Ca pyroxene versus Fa concentrations of ZnO (>2 wt%). content of olivine for mineral compositions in the MS-CH The CAI CH-2 is a complex spinel-rich aggregate and fragment compared with those for equilibrated (types 4–6) consists of three different units (Fig. 4b). The spinel has ordinary chondrites. Note that in the case of the Almahata considerable concentrations of FeO and ZnO of 20 and Sitta fragment, the olivines are almost equilibrated and that the pyroxenes have a weak maximum at Fs24–26, and show a 3 wt%, respectively (Table 3). The Cr concentration wide range of compositions (see text). Figure after Keil and increases significantly toward the adjacent chondrite Fredricksson (1964), Sears and Dodd (1988), and Brearley and matrix in one area of the inclusion having no Jones (1998). Ca-pyroxene rim (Fig. 4b). Here, the spinel has FeO, Cr2O3, and ZnO concentrations of 30, 35.5, and representative analyses of the constituents are given in 1.5 wt%, respectively (Table 3). In other areas, the Table 3. spinel cores are surrounded by Na- and Cl-rich alteration products, probably sodalite (Table 3). Within the spinels, Matrix tiny inclusions of ilmenite and FeS were found as The matrix is fine grained and strongly dominated well as an Os-,Ru-bearing Fe,Ni-metal grain. Some by olivine (Fig. 4c). The olivine is rather uniform in Unique chondritic fragment Almahata Sitta MS-CH 1661

Fig. 4. Backscattered electron micrographs of calcium-aluminum-rich inclusions (CAIs) in the MS-CH fragment. a) CAI CH-1 is a concentric spinel-rich inclusion. A spinel-rich core (Sp) is surrounded by a porous nepheline-bearing layer (Ne), an almost complete rim of Ca-pyroxene (Cpx), and in some areas by an additional rim of olivine (Fa36); Cr-Sp = Cr-spinel. b) CAI CH- 2 is a complex spinel-rich aggregate with three different units. The Cr concentration of spinel (Sp) increases significantly toward the chondrite matrix (Cr-Sp; lower left). Spinel cores are surrounded by Na- and Cl-rich alteration products, probably sodalite (So). Within the spinels, tiny inclusions of ilmenite (Ilm) and FeS were found as well as an Os-,Ru-bearing Fe,Ni-metal. Note the porous olivine-rich matrix surrounding the CAI. An = anorthite. c) The fine-grained matrix is strongly dominated by Fa-rich, mostly anhedral olivine locally intergrown with patches of plagioclase (An5–10); Cr = chromite, M = metal. d) The distribution of metals (T = ) and sulfides (FeS) within fragment MS-CH. Taenite is often enclosed within . Olivine (Ol; gray) and zoned low-Ca pyroxene (Px; dark to medium gray) dominate the silicate matrix; Ilm = ilmenite.

Table 2. Representative microprobe analyses (except as indicated) of metal and troilite in Almahata Sitta MS-CH. Metal Sulfide Taenite Kamacite* Troilite Si n.d. <0.01 n.d. <0.01 <0.14 <0.06 n.d. n.d. n.d. n.d. n.d. S n.d. <0.01 <0.02 0.07 n.d. n.d. 36.0 36.0 36.4 36.3 35.9 Ca <0.02 <0.03 n.d. 0.04 n.a. n.a. n.d. 0.05 <0.01 n.d. n.d. Cr <0.02 <0.04 0.14 n.d. n.d. <0.03 n.d. n.d. 0.05 n.d. 0.04 Mn n.d. n.d. n.d. n.d. n.d. n.d. 0.11 <0.01 n.d. <0.03 <0.01 Fe 56.8 57.7 57.1 75.9 91.3 91.4 61.8 62.5 62.6 62.4 62.3 Ni 40.9 40.4 40.6 21.6 6.8 6.9 1.18 0.15 n.d. 0.22 0.88 Cu 0.13 0.11 0.26 <0.04 n.a. n.a. n.d. 0.09 0.15 <0.03 n.d. Zn <0.04 0.19 0.08 0.12 n.a. n.a. <0.04 0.18 <0.03 0.07 n.d. Co 1.75 1.76 1.80 1.74 1.71 1.59 0.31 0.14 0.06 0.17 0.12 Total 99.66 100.25 100.00 99.52 99.95 99.98 99.44 99.12 99.30 99.22 99.25 Note: All data in wt%. n.d. = not detected; n.a. = not analyzed. *EDS analyses normalized to 100%. 1662 M. Horstmann et al.

Table 3. Representative microprobe analyses of phases within two calcium-aluminum-rich inclusions (CAI 1 and CAI 2). Nepheline-dominating alteration product Spinel Ca-Px Sodalite Spinel Cr-spinel CAI 1 CAI 1 CAI 1 CAI 1 CAI 2 CAI 2 CAI 2 CAI 2 CAI 2 CAI 2 CAI 2

SiO2 41.9 0.06 0.04 54.4 38.9 38.5 0.04 <0.03 <0.02 <0.03 0.19 Al2O3 30.2 64.3 63.9 0.82 31.5 30.8 63.4 63.1 62.9 63.2 26.8 Cr2O3 0.07 0.21 0.41 0.09 0.05 0.47 0.22 0.26 0.47 0.24 35.5 TiO2 0.20 0.04 0.06 0.05 <0.01 0.16 0.06 <0.03 0.06 0.12 0.77 FeO 0.92 20.5 20.6 1.28 0.84 1.81 19.8 20.4 20.5 20.3 29.9 NiO <0.03 0.05 0.05 <0.02 <0.01 n.d. 0.09 <0.03 0.05 0.05 n.d. ZnO 0.07 2.20 2.53 <0.03 0.07 0.12 3.4 2.40 2.77 2.64 1.52 MgO 1.44 12.8 12.4 19.0 0.11 1.13 12.5 12.7 12.8 12.5 4.3 MnO <0.03 0.12 0.16 <0.02 n.d. n.d. 0.11 0.15 0.11 0.08 0.36 CaO 2.47 0.07 0.08 23.4 0.19 0.31 <0.02 <0.02 n.d. 0.04 0.05 K2O 1.53 n.d. <0.03 <0.03 0.45 0.10 n.d. <0.01 <0.03 <0.03 <0.02 Na2O 16.6 0.19 0.10 <0.03 20.4 17.1 0.25 0.17 0.21 0.26 0.20 Cl 2.88 <0.02 n.d. n.d. 8.4 8.4 <0.03 <0.02 n.d. <0.03 n.d. Total 98.28 100.54 100.36 99.1 100.91 98.90 99.87 99.18 99.87 99.43 99.59 Fs ⁄ En ⁄ 2.0 ⁄ 52.0 ⁄ Wo 46.0 Note: All data in wt%. n.d. = not detected.

composition (Fa36); some grains are lath-like, but Sitta fall in October 2008. Data are presented in most are anhedral. The olivine grains do not show any Table 4. indication (e.g., zoning, tiny metal inclusions) that they have suffered reduction at grain boundaries. In parts of DISCUSSION the matrix, patches of plagioclase (An5–10) occur intergrown with olivine (Fig. 4c). Small particles of Classification and Origin of Almahata Sitta Fragment chromite, ilmenite, troilite, and metal are additional MS-CH distinct constituents of the matrix. Some representative analyses of the plagioclases and oxides are given in The main mineral olivine is almost equilibrated and Table 1. only a small number of large zoned grains exist; olivine is mainly Fa35–37. Matrix olivine is equilibrated and has Oxygen Isotopes a similar composition compared with most olivine within chondrules and fragments (Fa36). On the other Three aliquots were used for oxygen isotope hand, low-Ca pyroxenes are much more heterogeneous analyses. The fragments MS-CH-1 to -3 had d17Oof in composition than olivine, because equilibration of +4.6&, +5.1&, and +3.4& and d18O of +5.5&, olivine is significantly faster than that of pyroxene. +5.6&, and +3.7&, respectively. The corresponding Based on the compositional data of olivine and D17O values were +1.7&, +2.1&, and +1.4&. The pyroxene, the MS-CH chondrite is a high-grade type 3 mean d17O is +4.35&, the mean d18O is +4.94&, and chondrite. Considering the olivine analyses and the the mean D17O is +1.76& (Fig. 5). Thus, the mean deviation from the mean Fa content (35.0 ± value is similar to values for bulk R chondrites. 3.6 mole%), fragment MS-CH is of type 3.8. The higher type 3 petrologic type is also indicated by a slightly Cosmogenic Radioisotopes translucent fine-grained matrix instead of an opaque matrix found in type 3 chondrites of lower petrologic The determination of the short-lived cosmogenic type. As discussed in the following paragraph, radioisotopes in MS-CH revealed the detection of 46Sc Almahata Sitta fragment MS-CH is mineralogically (half-life: 83.788 days), of 54Mn (half-life: 312.13 days) different to rocks known from other chondrite groups and 57Co (half-life: 271.8 days). The value for 46Sc and must therefore be considered as a unique, new type clearly indicates that the fragment MS-CH results of chondritic rock. from a very recent meteorite fall, 335–420 days prior Based on the oxygen isotopes, Almahata Sitta to the measurement, consistent with the Almahata fragment MS-CH should be classified as a R chondrite Unique chondritic fragment Almahata Sitta MS-CH 1663

Mineralogical Similarities of Fragment MS-CH to Other Chondrite Groups

Based on the mineral compositions, chondrule sizes, and matrix abundances, some similarities exist between the Almahata Sitta fragment MS-CH and LL-group ordinary chondrites, CK chondrites, and Rumuruti (R) chondrites. These similarities and the differences will be discussed in the following: 1. Similarities to and differences from LL chondrites. Among the ordinary chondrites, the LL chondrites are the most oxidized group having high Fa- and Fs contents in olivine and pyroxene (roughly 27–32 and 22–25 mole%, respectively). Olivine in fragment MS-CH is nearly equilibrated and has about 36 mole% Fa, which is far above the value for LL-group ordinary chondrites. On the other Fig. 5. Oxygen three-isotope plot (d17O versus d18O) of hand, the Co concentrations in metals are very analyses of three fragments (small circles) and the mean (large similar to those in LL-group ordinary chondrites. diamond) of Almahata Sitta fragment MS-CH. The fields for Kamacite in LL chondrites has mean Co contents R and ordinary chondrites (H, L, and LL) are shown for reference. CCAM = anhydrous above approximately 1.4 wt% and taenite has up mixing line; TFL = terrestrial fractionation line. to 2 wt% Co (e.g., Afiattalab and Wasson 1980; Brearley and Jones 1998). A relationship between Almahata Sitta fragment MS-CH and LL-group Table 4. Data summary for the detected cosmogenic ordinary chondrites is precluded by the high matrix radionuclides in the sample MS-CH of the Almahata abundance of about 45 vol% and the chondrule Sitta meteorite. The reported uncertainties in the last diameter of about 450 lm. The matrix abundance digits (in parentheses) are expanded uncertainties with in ordinary chondrites is about 10–15 vol% and LL k =1. chondrites have mean chondrule sizes on the order Activity concentrations )1 of 900 lm (e.g., Grossman et al. 1988 and Radionuclide Half-life in [dpm kg ] references therein). However, more recently Nelson 26Al 717000 yr 57 (12) and Rubin (2002) determined a revised mean 60 Co 5.2710 yr 22 (5) chondrule size for LL chondrites of 570 lm. 54Mn 312.13 days 114 (19) 22 2. Similarities to and differences from CK chondrites. Na 2.6027 yr 78 (15) CK chondrites are commonly metamorphosed and 46Sc 83.788 days 19 (8) have equilibrated olivine with compositions of 57Co 271.8 days 22 (10) approximately Fa28–34 (e.g., Kallemeyn et al. 1991; Geiger and Bischoff 1995). Thus, the olivine (Fig. 5). The analyses of the fragments, however, show compositions of CK chondrites are very similar to a considerable scatter with the average falling toward those in Almahata Sitta fragment MS-CH; the lower d17O part of the R chondrite field, close to however, olivine in MS-CH does not contain the field of LL chondrites. The small amount of sample significant NiO as observed in CK chondrites. In material analyzed (<5 mg) may also not be entirely addition, the fragment studied here contains about representative for the bulk composition of the fragment. 2.5 vol% metals. This is not the case for CK The heterogeneity in oxygen isotope composition shows chondrites, which typically contain abundant that isotopic equilibration of Almahata Sitta fragment magnetite (e.g., Geiger and Bischoff 1995). The MS-CH has not been reached. chondrule size of approximately 450 lm in MS-CH The detection of the short- and medium short-lived is also significantly smaller than that found for CK cosmogenic radionuclides 46Sc, 57Co, and 54Mn clearly chondrites (close to 1 mm; Kallemeyn et al. 1991). indicates that the chondritic fragment MS-CH results In addition, fragment MS-CH does not contain from a recent meteorite fall consistent with the Asteroid magnetite and PGE-rich sulfides, tellurides, and 2008 TC3 event in October 2008. Therefore, we have arsenides typically present in CK chondrites. strong evidence that fragment MS-CH belongs to the 3. Similarities to and differences from Rumuruti (R) Almahata Sitta meteorite fall. chondrites. The equilibrated Rumuruti (R) chondrites 1664 M. Horstmann et al.

have homogeneous Fa contents in olivine of about metamorphic processes and ⁄ or by interactions in the 36–40 mole% (e.g., Bischoff et al. 1994; Rubin and solar nebula. They all contain considerable Fe, Zn, and Kallemeyn 1994; Schulze et al. 1994; Kallemeyn Cr in spinel and have ilmenite instead of perovskite. et al. 1996; Rout and Bischoff 2008). This is in the The formation of nepheline and ⁄ or sodalite in the outer same range as olivine within the Almahata Sitta areas of the CAIs (Fig. 4) may be due to nebular fragment MS-CH (Fa35–37). Rubin and Kallemeyn alteration as also found in other chondrite groups. It (1993) characterized the R chondrites as having should be mentioned that other scientists do not regard 42 ± 11 vol% matrix, besides chondrules and nepheline as being a nebular product in (e.g., large mineral and lithic fragments. Bischoff (2000) Krot et al. 1995; Russell et al. 1998; Tomeoka and Itoh estimated the chondrule-to-matrix ratio in unequili- 2004), but formed during alteration. brated type 3 fragments of R chondrites to be Melilite has probably been replaced by nepheline ⁄ approximately 1:1. The chondrule size of R sodalite. chondrites is on the order of 400 lm (e.g., Bischoff It is also known from other somewhat et al. 1994; Rubin and Kallemeyn 1994; Schulze metamorphosed, high-grade type 3 or type 4 ordinary et al. 1994; Kallemeyn et al. 1996; Imae et al. 1999; chondrites, R chondrites, CV, CO, and CK chondrites Rout and Bischoff 2008). Thus, the matrix that CAIs often have ilmenite instead of perovskite and abundance and the chondrule size of R chondrites Fe-rich spinel (e.g., Bischoff and Keil 1983a, 1983b, are in the same range as those of the Almahata Sitta 1983c, 1984; Geiger et al. 1993; Brearley and Jones 1998 fragment MS-CH. A relationship to the R chondrites and references therein; Bischoff and Schmale 2007; is also indicated by similar oxygen isotope Rout and Bischoff 2007, 2008). CaO in perovskite is compositions. But distinct differences exist: R replaced by FeO to form ilmenite during secondary chondrites do not contain abundant metals as alteration. Similarly, Mg in spinel is replaced by Fe and observed in MS-CH. The very few metals described Zn. Considering the replacement processes within from R chondrites usually exist enclosed and spinels and perovskites, it is suggested that the chemical protected within forsteritic olivines that occur mainly exchange processes started in the nebula (in many in chondrules of type 3 lithologies. Olivines in R chondrites, CAIs containing ilmenite coexist with CAIs chondrites have significant NiO in their structures, containing perovskite), followed by chemical alteration which is not the case for olivines of the Almahata by diffusion during parent body metamorphism. In Sitta fragment (Table 1). R chondrites typically analogy to the spinels studied here, Fe-rich spinel is a contain PGE-rich sulfides, tellurides, and ⁄ or typical mineral in CAIs from the metamorphosed CK arsenides, which are lacking in fragment MS-CH. chondrites as a result of metamorphic processes (e.g., Also, the Cr-spinels have much lower TiO2 MacPherson and Delaney 1985; Noguchi 1993; Geiger concentrations than those within R chondrites. These and Bischoff 1995). characteristics also clearly speak against a common origin of the fragment MS-CH and R chondrites. The Significance of In summary, Almahata Sitta fragment MS-CH is mineralogically clearly different to rocks known from To reveal information on the evolution of asteroidal other chondrite groups. Therefore, it has to be parent bodies, it is very important to study meteorite considered as a unique, new type of chondritic rock. breccias (e.g., Bischoff et al. 1983, 1993, 2006; Sto¨ ffler However, based on some mineralogical and isotopic et al. 1988). Especially, the existence and abundance of aspects (oxygen isotopes), a certain affinity to R foreign and exotic fragments in certain meteorites give chondrites may exist. fundamental information on the degree of mixing among in the asteroidal belt. The mixing of Secondary Alteration by Metamorphism and ⁄ or Nebular fragments of different types in chondrites and Processes is summarized in Bischoff et al. (2006). In this respect, Almahata Sitta is a complex breccia As stated above, olivine is almost equilibrated and offering potentially new and interesting xenolithic only a small number of large zoned grains exist. Olivine fragments (Bischoff et al. 2010a). Previous reports on in chondrules and fragments is mainly Fa35–37, similar polymict show that in composition to equilibrated matrix olivine (Fa36). fragments (e.g., Jaques and Fitzgerald 1982; Prinz et al. Based on the compositional data of olivine, the MS-CH 1986, 1987, 1988; Ikeda et al. 2000, 2003; Ross et al. chondrite is quite equilibrated due to parent body 2010) and -like clasts have been found within metamorphism and is a high-grade petrologic type 3 several meteorites (e.g., Jaques and Fitzgerald 1982; chondrite. The CAIs have also been altered by Prinz et al. 1986, 1987; Ikeda et al. 2000; Goodrich and Unique chondritic fragment Almahata Sitta MS-CH 1665

Keil 2002; Cohen et al. 2004; Kita et al. 2004). In constructive reviews and the associate editor Adrian addition, fine-grained dark clasts, mineralogically Brearley for thoughtful comments. similar to fine-grained carbonaceous chondrite material, Editorial Handling—Dr. Adrian Brearley are known to occur in some ureilites (e.g., Prinz et al. 1987; Brearley and Prinz 1992; Ikeda et al. 2000, 2003; Goodrich and Keil 2002). Thus, chondritic clasts in REFERENCES ureilites are well-known; however, the huge variety of Afiattalab F. and Wasson J. T. 1980. Composition of metal different chondrites found in Almahata Sitta is phases in ordinary chondrites: Implications regarding exceptional. These clasts are characterized and classification and metamorphism. Geochimica et summarized in detail by Bischoff et al. (2010a). In this Cosmochimica Acta 44:431–446. respect, one of the most complicated meteorite breccias Armstrong J. T. 1991. 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